docs: add Triangle and estimator I/O examples (#704)

[EKtheSage]

Summary of Changes

• Add doctest Examples for Triangle.to_pickle / Triangle.to_json and the estimator mixins to_pickle / to_json in chainladder/core/io.py.
• Examples round-trip through read_pickle / read_json so the write methods are shown in the same pattern as the already-documented readers.

Related GitHub Issue(s)

• API Reference Examples #704

Additional Context for Reviewers

Bite-sized follow-up to the remaining core-module work on #704. Companion PR for slicing: #1208.

Checklist

• I passed tests locally for both code (uv run pytest) and documentation changes (uv run --directory docs jb build . --builder=custom --custom-builder=doctest)

Made with Cursor

[github-actions]

Pyright Type Completeness

View the full pyright --verifytypes output for this commit

Project (full chainladder package, at this PR's head): 15.1% of exported symbols fully typed (201 / 1333)

	Known	Ambiguous	Unknown	Total
Project (head)	201	111	1021	1333

Other symbols referenced but not exported by chainladder: 13

	Known	Ambiguous	Unknown	Total
Other (head)	3	1	9	13

Symbols without documentation:

• Functions without docstring: 321
• Functions without default param: 0
• Classes without docstring: 10

Patch (exported symbols added or changed by this PR): no exported symbol type-completeness changes detected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.25%. Comparing base (52e655b) to head (083bd6a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1209      +/-   ##
==========================================
- Coverage   91.26%   91.25%   -0.01%     
==========================================
  Files          91       91              
  Lines        5401     5400       -1     
  Branches      691      691              
==========================================
- Hits         4929     4928       -1     
  Misses        338      338              
  Partials      134      134              

Flag	Coverage Δ
unittests	91.25% <100.00%> (-0.01%)	⬇️

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[henrydingliu]

I'm good with the docstring changes. do you mind addressing the ruff beef while you are here?

[EKtheSage]

The ruff workflow runs with lint.per-file-ignores={}, so touching io.py surfaced the grandfathered E731 (lambda assigned to j in EstimatorIO.to_json).

Replaced it with an inline dict comprehension and dropped the chainladder/core/io.py ignore from pyproject.toml.